The study of antibodies to staphylococcal nuclease has been extended in two directions. First synthetic peptides corresponding to regions of the primary structure of nuclease have been synthesized and coupled to solid supports. These peptide supports were used to isolate antibodies specific to the regions being studied. The association constants were determined. Second, peptide fragments available from cyanogen bromide cleavage and limited trypsin digestion were used to isolate an antibody population specific for amino acid region corresponding to the sequence 99-126. The interaction of this population with nuclease T', an enzymatically active complex of two trypsin fragments of nuclease, is being studied kinetically by radiolabelling of the fragments. This approach allows one to measure directly the dissociation constant of nuclease T' from anti 99-126.